1. Field of the Invention
The present invention relates to a stereoscopic-vision endoscope system which can be used for medical treatment and for industry and which electrically corrects distortion of an image with respect to left- and right-hand image signals having a parallax, independently of each other, such that left- and right-hand variations are so dissolved as to be able to practice stereoscopic vision.
2. Description of the Related Art
In recent years, in an endoscope for medical treatment which is used in observation and therapy of, chiefly, the stomach, the large intestine, the bronchus and the like, and, in an endoscope for industry which is used in observation of pipes such as gas pipes, water pipes or the like, and for internal observation of engines or the like, there has been remarkable improvement in image quality. Particularly, various solid-state image pickup elements, such as a charge coupled device (hereinafter, referred to as "CCD"), has advanced miniaturization and produces a high image quality of an electronic endoscope in which the CCD is disposed at a forward end thereof. In keeping therewith, there is a need for increased stero image observation in endoscopes used for medical treatment, where surgical operation is practiced under the endoscope, and, in case, in the endoscope for industry, where the magnitude and the depth of each of flaws, cracks or the like are measured, and the like.
In other fields, stereo observation technique has already been advanced, and it is considered that this technique is applied to the endoscope. As the technique of the stereo observation, a stereoscopic-vision observation system has been developed in which two television cameras are used, two slightly-different images which are generated by a difference in positions of right- and left-hand cameras, that is, a parallax, is acquired to thereby fetch the stereoscopic-view image, and these images are presented on both eyes of a human being, for example, independently of each other, to thereby produce a stereoscopic sense of perspective.
Methods of presenting the stereoscopic-vision image to the human being include a method shown in FIGS. 1A to 3. A method shown in FIGS. 1A-1C is formed such that a monitor 121L for a left-hand eye and a monitor 121R for a right-hand eye are respectively arranged in opposed relation to left- and right-hand eyes 122L and 122R.
In FIG. 1A, for example, the arrangement is such that images on the monitors 121L and 121R are reflected respectively by two mirrors 123L and 124L and 123R and 124R and are seen respectively by left and right eyes 122L and 122R.
The arrangement in FIG. 1B is such that the images on the monitors 121L and 121R are refracted to the left and the right respectively by prisms 125L and 125R and are observed. Further, the arrangement in FIG. 1C is such that the image on the monitor 121L is observed by the left eye 122L, and the image on the monitor 121R is reflected by a mirror 126 and is observed by the right eye 122R.
The arrangement in FIG. 2 is such that polarizing plates 127L and 127R which are set in a perpendicular polarizing direction are arranged respectively in front of display surfaces of the monitors 121L and 121R for the left eye and the right eye, polarizing plates 128L and 128R which are set in the perpendicular polarizing direction are disposed respectively also in front of the left and right eyes 122L and 122R, and the images on the monitors 121L and 121R are seen respectively by the left and right eyes 122L and 122R through a half mirror 129.
FIG. 3 shows that a single monitor 121 is used; and a polarizing member 129 capable of varying the polarizing direction is disposed in front of the monitor 121. The polarizing plates 128L and 128R which are set in the perpendicular polarizing direction are disposed respectively also in front of the left and right eyes 122L and 122R. In the case where left-hand image data of a left-hand-image-data output circuit 130L are displayed on the monitor 121, the polarizing direction of the polarizing member 129 is set to the same direction as the polarizing plate 128L by a left- and right-hand changeover circuit 131; and, in the case where right-hand image data of a right-hand-image-data output circuit 130R are displayed on the monitor 121, the polarizing direction of the polarizing member 129 is set to the same direction as the polarizing plate 128R by the left- and right-hand changeover circuit 131, whereby the left- and right-hand images which are displayed on the single monitor 121 can be observed in separation by the left and right eyes 122L and 122R.
In addition to the above, there is a method of separating images from each other by the use of an electronic shutter.
A prior art example of a stereoscopic-vision system 132 using a single monitor is shown in FIG. 4. A camera 133L for a left-hand image and a camera 133R for a right-hand image respectively photograph images having a parallax between both eyes. The images which are acquired by the left- and right-hand cameras 133L and 133R become digitized image signals, respectively through A/D converters 134L and 134R, and are recorded onto frame memories 135L and 135R. Recording signals thereof are read out alternatively by a double-speed stereo converter 136. Double-speed stereo image signals thereof are converted respectively to analog signals by a D/A converter 137 and are projected onto a double-speed scan monitor 138. Liquid-crystal spectacles 139 are arranged such that left- and right-hand shutters are alternately opened and closed synchronously with the double-speed scan monitor 138.
Accordingly, the left-hand image is seen only by the left eye of an observer, while the right-hand image is seen only by the right eye. Thus, because of the parallax of both eyes, the observer can perceive the image as a stereo image. There is also an arrangement which utilizes polarization in place of the liquid crystal.
Moreover, in order to raise the accuracy of the stereo image, as disclosed, for example, in Japanese Patent Unexamined Publication No. HEI 4-108288 (108288/1992), two television cameras are disposed such that normals of respective image pickup surfaces thereof are parallel to each other, signals are processed such that an interval between image pickup points corresponding respectively to image centers of the two image pickup surfaces is greater than an interval between the centers of two lenses, and lines tying the centers of the lenses and the image pickup points corresponding, respectively, to the image centers to each other are intersected with each other in front of the stereo camera. Since the image pickup surfaces are disposed in parallel to each other, and since the image signals of the two cameras can be shifted in parallel to adequate locations, accurate stereoscopic vision can be practiced which has no distortion.
By the way, the most important matter in the stereoscopic vision which is practiced by the use of the two cameras is that no mutual variation exists between the two cameras. Examples of the variation include image falling-down which occurs when the image pickup surface (CCD surface, for example) of one of the cameras is rotated with respect to a center of an optical axis; inconsistency of the magnitude of the image due to the fact that focal distances of respective lens systems are slightly different from each other and positional shift between the left- and right-hand images due to positional shift of the CCD; distortion of the image due to eccentricity of partial lenses of a lens system and the like.
If the variation occurs mutually, it is impossible to acquire a correct stereo image, and correction is applied by eyes of the observer whereby, if observation is practiced for a long period of time, the observer is liable to be very tired. For this reason, in order to correct the mutual variation, the camera of the prior art example uses a lens barrel which is provided with a CCD position adjustment mechanism and a lens adjustment mechanism, to practice mutual adjustment.
By the way, it is desired for the endoscope to observe a wide range at a time. A minute or microscopic objective lens which has a wide angle-of-view as compared with a general television camera (equal to or more than 60.degree.-140.degree.) and, further, which is disposed at a forward end of an insertion part having a narrow or reduced diameter is used. Accordingly, a variation resulting from working of the lens unit and a variation resulting from assembling of lens parts increase relatively more than in the case where large objective lenses are used.
It is desirable that the forward end of the endoscope is reduced in diameter as much as possible in order to reduce pain resulting from insertion into a patient, or the like. For this reason, the objective lens which is disposed at the forward end must be made as simple a structure as possible. It is difficult to provide a complicated adjustment or regulation mechanism.
In view of the above, an arrangement is proposed in Japanese Patent Unexamined Publication No. HEI 6-59196 (59196/1994) in which shift between left- and right-hand object images is electrically corrected. However, this is arranged such that amounts of shift of the left- and right- hand images are detected from a shift between outlines of the respective images, or the like, and readout addresses of the respective images which are temporality stored in a memory are controlled, or the like, to correct such that both the images coincide with each other.
In this prior art example, a relative amount of shift between the left- and right-hand images is detected to correct such that both the images are coincided with each other, from the amount of shift. Accordingly, principally, correction is carried out such that one of the images is made to coincide with the other image. Thus, in the case where vertical directions of the CCDs are both inclined with respect to a vertical direction of an endoscope body, it is difficult for this prior art example to correct the inclination. Further, also in the case where the magnitudes of the left- and right-hand images are different from each other caused by variations or the like of the left- and right-hand objective lenses, it is impossible to solve this problem by mere correction of the addresses. Similarly, also in the case where distortion of the images due to eccentricity or the like of an optical system is generated on both objective lens systems, correction is carried out on the basis of the relative amount of shift between both the distortions. Accordingly, it is impossible to correct distortion portions which are commonly generated. Thus, there is a problem that it is impossible to acquire a natural stereo image which is arranged so as to faithfully reproduce an object.
Moreover, since there are few degrees of freedom of the layout of the forward end, it is difficult to acquire a parallax which is optimum or adequate to practice stereo observation. What is meant by the optimum parallax means the relation in which inward angles .THETA. of the respective two cameras 133L and 133R, for example, satisfy 2.degree.&lt;.THETA.&lt;20.degree. with respect to an observation object S shown in FIG. 5. If the inward angle .THETA. is equal to or less than 2.degree., a stereo sense or a three-dimensional feeling cannot be acquired.
If the inward angle .THETA. exceeds 20.degree., the left- and right-hand images form double images so that ocellus suppression is generated such that it is impossible to recognize the left- and right-hand images as a single image, and that only a picture on one side is seen. Thus, a deficiency also occurs such that it is impossible to carry out the stereo observation.
In a prior-art general stereo camera, movement means (not shown) is provided so as to be able to set a desired interval between two television cameras, the interval between the cameras is adjusted in accordance with the distance with respect to an observation object, and the inward angle is determined so that a proper parallax is acquired.
In the endoscope, however, it is almost impossible for such movement means to be provided. The interval between the objective lenses corresponding to the two cameras is determined on the basis of the relationship between the inward angle and the distance of the practical range to the observation object. Specifically, the inward angle upon accession increases, while the inward angle upon observation of a great distance decreases. In case of the endsocope for medical treatment, however, it is desired that the interval between two cameras be set such that the inward angle is equal to or less than 20.degree. upon accession or proximity. Assuming that an object distance to the observation object upon accession shown in FIG. 5 is Lnf, and the interval between the two cameras is D, then, the interval D must be set that the following relationship is held: EQU (D/2).times.(1/Lnf)=tan (.THETA./2) EQU D&lt;0.35Lnf (because of .THETA.&lt;20.degree.)
If for example, it is supposed taht the distance Lnf upon accession is 10 mm, the distance D between the two cameras must be made to a value equal to or less than 3.5 mm. Here, whenever a high image quality of the stereoscopic vision is desired, it is necessary to provide multiple picture elements of the CCD so that teh size of the CCD increases. Then, it is necessary to increase the objective lens interval such that an interference of a package of the CCD does not occur. For this reason, it becomes difficult in the endoscope of the prior art example to satisfy the above-described relationship.